The electrical current-carrying capacity of a superconductor is determined by its critical current Ic. If the electrical current in the conductor exceeds the value of Ic, a phase transition converts the superconductor back to a normal state in which the current no longer flows without resistance.
In an isotropic superconductor, the current-carrying capacity depends on the strength of the magnetic field to which it is exposed, but not on the direction of the magnetic field. In contrast, in an anisotropic superconductor, the current-carrying capacity is also influenced by the angle of the magnetic field relative to the superconductor. This is true of high-temperature superconductors (HTSs), for example, (RE)BCO or Bi-2223, the underlying copper oxide structure of which has an anisotropic, two-dimensional character. The critical current of an HTS strip conductor in a magnetic field perpendicular to the strip plane is therefore typically lower than in a magnetic field parallel to the strip plane.
In a cylindrically symmetrical magnet coil of wound HTS strip conductors, this normally results in the current-carrying capacity of the coil being limited at the axial ends because the radial component of the magnetic field is greatest at the axial ends.
In some example of prior art coils (e.g., U.S. Pat. No. 5,525,583 and U.S. Pre-Grant Publication No. 2015/0213930), the current-carrying capacity of the coil at the axial ends is increased by using a superconductor for the corresponding windings which has a higher current-carrying capacity (e.g., a larger conductor cross-section, or a type of superconductor which has a higher critical current density). One disadvantage of this solution is that it is not possible to use a single type of superconductor in the coil, and the different conductor pieces must necessarily be connected in series with low resistance to operate. In addition, many prior art coils do not consider layer wound coils. They only consider coils which consist of multiple sections or pancakes positioned axially along the axis.
A further known option (e.g., U.S. Pat. No. 5,659,277) for increasing current-carrying capacity relies on ferromagnetic flanges on the ends of the coil to guide the magnetic flux around the superconductor, locally reducing the maximum radial component of the magnetic field. However, the relatively weak magnetization of ferromagnets significantly limits the efficiency of this method.
Another prior art coil (e.g., U.S. Pat. No. 5,581,220) discloses an arrangement in which the number of windings is reduced on the axial coil ends. However, this known coil is an arrangement of multiple double-pancake coils, and not a layer wound solenoid coil as described herein. In addition, this arrangement is not intended to reduce the radial field components at the ends of the coil.
Additional prior art solutions (e.g., “Factors determining the magnetic field generated by a solenoid made with a superconductor having current anisotropy”, M. Däumling and R. Flükiger, (1995) Cryogenics, Vol. 35. pp. 867-870; “Effects of conductor anisotropy on the design of BiSCCO sections of 25 T solenoids”, H. W. Weijers et al. (2003), Supercond. Sci. Technol. Vol. 16, pp. 672-681; and “Radial magnetic field reduction to improve critical current of HTS solenoid”, J. Kang et al, Physica. C., 2002, vol. 372-76 (3), pp. 1368-1372) recognize that it is possible to increase the operating field in a working volume by reducing the radial field at the edge of an HTS coil, of these prior art solutions suggest coils of different lengths to reduce the radial field. However, the operating field increase in the working volume which results from this measure is small. Moreover, an additional winding body is necessarily required in each of the known arrangements.
Another prior art coil (e.g., German Publication No. DE 102004043987 B3 discloses a superconductive magnet coil arrangement having at least one section made of a superconductive strip conductor which is continuously wound in a cylindrical winding chamber between two end flanges in multiple, solenoid-like layers. This prior art coil is characterized in that the section has an axial region of reduced current density or a notch region.
Yet other prior art coils (e.g., German Publication No. DE 39 23 456 C2) describe a superconducting, homogenous, high field magnet coil in which the current density in the axial end region is reduced in such a manner that the forces acting on the windings can be kept low.
Still other prior art coils (e.g., German Publication No. DE 10 2004 043 988 B3) disclose a superconductive magnet coil arrangement having at least one section made of a superconductive strip conductor which is continuously wound in a cylindrical winding chamber between two end flanges in multiple, solenoid-like layers. The known arrangement is characterized in that the section has an axial region of reduced current density (a notch region). However, the number of windings at the coil edges compared to the interior of this axial region is not reduced. As a result, no reduction of the radial field is achieved.
Another coil geometry (e.g., described in JP H06-5 414 A) shows the inner diameter of the windings on the coil edge expanding in order to reduce the influence of the vertical field components on the critical current density. In this arrangement, among other things, the inner coil radius is varied axially, which for various reasons is not particularly advantageous and is diametrically opposed to the corresponding feature of a coil in the class. In addition, the co-winding of non-superconducting material in a layer wound coil with cylindrical symmetry about the axis of symmetry z is not disclosed.
Another prior art coil geometry (e.g., CHEN, X. Y., JIN, J. X.: Evaluation of Step-Shaped Solenoidal Coils for Current-Enhanced SMES Applications. IEEE Transactions an Applied Superconductivity, Vol. 24, 2014, No. 5, S. 1-4. IEEE Xplore [online]. D01: 10.1109/TASC.2014.2356572) describes a superconductive magnet coil arrangement in the class, having the some of the features described herein. However, the coil described in the prior art is formed exclusively from pancake coils, and not layer wound coils with cylindrical symmetry about the axis of symmetry, and no co-winding of non-superconducting material is disclosed.